Creation Model of the Bostick Plasmoid in Magnetic-Field-Free Space

Abstract

A creation model has been developed to interpret the behavior of Bostick plasmoids in the plane of the discharge. The plasmoids are formed by pulsed unidirectional current discharges across the electrodes of a button gun in vacuo (10−6 Torr). The model is that of a vacuum arc which expands in the form of a thin, highly conducting torus by virtue of the interaction of the surge current with its self-induced magnetic field. When the current no longer flows, the plasma of the arc continues to move by virtue of its own inertia. The plasmoid speeds directly away from the button gun have been determined experimentally from time of arrival measurements to moveable electrostatic probes, and are found to increase monotonically from 20 to 100 km/sec for a corresponding source voltage range of 2.5–17.5 kV. The measured velocity ν′ of any point on the plasma which subtends an angle θ to the direction of forward motion at the button-gun source is given by v′=〈I〉(α/πρ)1/2cosθ, where 〈I〉 is the average surge current, ρ is the measured mass per unit length (line density) of the genetic plasmoid and α is a theoretically calculated constant. The line density ρ of the plasmoid has been shown theoretically to be a constant and its magnitude has been measured to be 10−9 kg/m and hence the mass M of the plasmoid is deduced theoretically to be given by: M=〈I〉τ(απρ)1/2, where τ is the duration of the current pulse. This formula predicts the plasmoid mass to increase from 0.05 to 0.2 μg for the current range of interest and the predicted mass is verified to within 16% by a novel experimental technique.